Yeah. The sales charts for popular and successful series I've seen basically have book two at about 50% of the sales of book one, and book three at about 30% of the sales of book one.

In the 1980s (as well as 70s to 90s) having a book series cancelled happened fairly frequently. At least one fantasy series per year, it seemed.

The usual cause was poor sales of later books in a series. Fans don't understand that for most readers, until they read book two, they are not going to buy book three (or later) in a given series. If book two or book three was released the month some other blockbuster was released, even cross-genre like Stephen King, that could be enough to kill an otherwise successful fantasy series. As books had to sell in the limited time that the store was willing to dedicate shelf space to a given title.

One reason is US publishers know that most of their books are sold online, and so lean heavily on making book covers that are good thumbnails. High colour contrast, large text title (or author's name), high impact imagery. Most European books look best as a physical object.

Covers that stand out in a media rich advertising storm of social media and online shopping are the "right" pick for advertising acclimated Americans.